Hi Audrey,
There was a bit of chat on underspent budgets here: How to use underspend - The SDS Forum - SDS Forum (sdsscotland.org.uk)
In general, if you can evidence that the money is being used to meet your unmet needs/outcomes then is should be perfectly acceptable. You could speak to your local independent SDS advice organisation, who may be able to support you to negotiate with the local authority around the underspend: Find Local Information and Support - Self Directed Support Scotland | Self Directed Support Scotland (sdsscotland.org.uk)
Regarding the driving lessons for your PA, I have not heard of this and the local authority might see it as the PA’s responsibility to ‘upskill’ themselves, but you can only ask, right?
It’s possible the PA could access support for training/developing their small business. The PA Network Scotland has just re-launched, so it might be worth your PA looking into what they can offer: PA Network Scotland
Hope this helps
Justin